A two-stroke, two-cycle, or two-cycle engine is a type of internal combustion engine which completes a power cycle in only one crankshaft revolution and with two strokes, or up and down movements, of the piston in comparison to a “four-stroke engine”, which uses four strokes. This is accomplished by the end of the combustion stroke and the beginning of the compression stroke happening simultaneously and performing the intake and exhaust (or scavenging) functions at the same time. (FIG. 2)
Two-stroke engines often provide high power-to-weight ratio, usually in a narrow range of rotational speeds called the “power band”. Compared to 4-stroke engines, they have a greatly reduced number of moving parts, are more compact and significantly lighter. Gasoline (spark ignition) versions are particularly useful in lightweight (portable) applications such as chainsaws and motorcycles. Two-stroke engines have higher exhaust emissions than four-stroke engines.
The lower cost to rebuild and maintain made the two stroke engine popular. However, due to recent concerns and regulations concerning environmental impact of these engines and the resulting emission regulations, the industry has largely to four-stroke petrol engines, which emit less pollution. Most small designs use petroil (two-stroke oil) lubrication, with the oil being burned in the combustion chamber, causing “black smoke” and other types of exhaust pollution. This is a major reason why two-stroke engines were replaced by four-stroke engines in many applications.
Simple two-stroke petrol engines continue to be commonly used in high-power, handheld applications such as string trimmers and chainsaws. The light overall weight, and light-weight spinning parts give important operational and even safety advantages. For example, a four-stroke engine to power a chainsaw operating in any position would be much more expensive and complex than a two-stroke engine that uses a gasoline-oil mixture.
Despite these problems, two cycle engines are still preferred for small, portable, or specialized machine applications such as outboard motors, high-performance, small-capacity motorcycles, mopeds, underbones, scooters, tuk-tuks, snowmobiles, karts, ultralights, model airplanes (and other model vehicles) and lawnmowers and dirt bikes.
Conventional two-cycle engines thus suffer from many drawbacks, including the mixture of oil and fuel incomplete combustion, which contributes to pollution inside and outside of the engine, as well as reduced, sub-optimal performances in power and speed.
There is a need for an engine that maintains the advantages of conventional two cycle engines, while addressing their deficiencies of high emission and pollution which are primarily caused by the combustion of oil and fuel mix in conventional two cycle engines.